This invention relates to a sintered non-oxide ceramic composite which has high density, high high-temperature strength and high toughness and electrical conductivity which can be optionally and widely changed.
Silicon carbide (SiC) is one of the promising materials as ceramics which are high in hardness, excellent in wear resistance and oxidation resistance, small in thermal coefficient of expansion and chemically highly stable. Further, silicon carbide high density sintered body is superior in high-temperature strength and thermal shock resistance and is exposed to be used for engine parts, gas turbines, etc. as structural materials along with silicon nitride (Si.sub.3 N.sub.4).
Furthermore, titanium carbide.nitride such as titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), etc. are also known as ceramics high in hardness and excellent in wear resistance, high in electrical conductivity and high in melting point. High density sintered bodies of titanium carbide.nitride are known to have high high-temperature strength and undergo plastic deformation at high temperatures, though inferior in oxidation resistance at high temperatures and are also expected to be applied as high-temperature structural materials like silicon carbide.
However, ceramic materials including SiC sintered body are generally brittle materials and low in toughness and are readily broken due to surface or internal micro-defects or abrupt stress concentration. Thus, they are poor in reliability.
Moreover, ceramic materials are generally high in hardness and, except for titanium carbide.nitride and the like, are difficultly workable materials because of low electrical conductivity and inferior electrical discharge workability. This problem in workability is one cause for delay in commercialization of ceramics.
In order to improve toughness of ceramic materials, for example, zirconia (ZrO.sub.2), there have been made researches to improve toughness by partial stabilization of zirconia by addition of yttrium oxide (Y.sub.2 O.sub.3), magnesium oxide (MgO), calcium oxide (CaO), etc. and the product has been known as partially stabilized zirconia (PSZ).
With reference to non-oxide ceramics including SiC, substantially few researches have been made for improvement of their toughness, but according to "J. Am. Ceram. Soc." Vol. 67, No. 8 (1984), pages 571-574, to silicon carbide (SiC) were added titanium carbide (TiC) and aluminum (Al) and carbon (C) as sintering aids to improve fracture toughness (K.sub.IC) of the resulting sintered body.
On the other hand, for improving electrical conductivity of silicon carbide (SiC), "Journal of the Japanese Ceramic Society", Vol. 93, No. 3 (1985), pages 123-129 discloses that to SiC is added titanium carbide (TiC), titanium nitride (TiN), zirconium carbide (ZrC), tungsten carbide (WC), zirconium boride (ZrB.sub.2), titanium boride (TiB.sub.2), hafnium boride (HfB.sub.2), niobium boride (NbB.sub.2), tantalum boride (TaB.sub.2) or the like at 50% by volume and is further added 2% by weight of Al.sub.2 O.sub.3 as a sintering aid for SiC and then the mixture is subjected to hot press sintering to sharply reduce resistiviy of the sintered body. Furthermore, Japanese Patent Kokai (Laid-Open) No. 196770/82 discloses addition to SiC of 0.5-30% by volume of at least one member selected from carbides, nitrides, borides and oxides of elements of Groups IVa, Va and VIa of the periodic table and aluminum carbide (Al.sub.4 C.sub.3), thereby to obtain sintered bodies improved in electical conductivity and superior in electric discharge workability. However, this publication makes no mention of sintering aids for SiC.
It is difficult to sinter SiC alone owing to its markedly high covalent bonding. Therefore, production of sintered composites having desirable characteristics such as high density, high strength, etc. becomes possible only with addition of a specific sintering aid to SiC. When aluminum (Al) or aluminum compounds are used as a sintering aid for SiC as reported in the above "J. Am. Ceram. Soc." and "Journal of the Japanese Ceramic Society", sintered bodies of high density can be obtained, but high-temperature strength at 1200.degree.-1500.degree. C. is extremely deteriorated.
According to Japanese Patent Kokai (Laid-Open) No. 112669/85, growth of crystal grains of SiC sintered body is prevented by adding 0.1-1.0% by weight of TiN to SiC with addition of boron or boron compounds and carbon or carbon compounds as a sintering aid. However, addition of TiN in such small amount of 0.1-1.0% by weight cannot afford sufficient increase of fracture toughness and besides, remarkable improvement of electrical conductivity cannot be expected.